Belt-type stepless transmission devices are already known and supplied for practical use comprising a drive pulley of variable width, a driven pulley of variable width, and a belt member that is engaged with this drive pulley and driven pulley. In such a transmission, there are provided a drive hydraulic actuator that performs pulley width control (axial thrust control) of the drive pulley and a driven hydraulic actuator that performs pulley width control (axial thrust control) of the driven pulley, thereby effecting pulley width setting control by controlling the axial thrust of the two pulleys by means of the hydraulic pressures that are supplied to these two hydraulic actuators, and so enabling the transmission ratio to be varied in stepless fashion.
Various transmission control devices have previously been proposed in order to control the hydraulic pressure supplied to such hydraulic actuators as reliably as possible in response to the running condition. Examples of such transmission control devices are disclosed in Japanese Laid-Open Patent Application H. 8-42652 and Japanese Laid-Open Patent Application H. 8-326857. These devices are arranged such that the pulley axial thrust balance for adjusting the transmission ratio is set by controlling the hydraulic pressure (axial thrust) acting on the driven pulley so as to confer on the pulley the minimum necessary pulley axial thrust (belt clamping force) such as to avoid belt slippage and controlling the hydraulic pressure (axial thrust) acting on the drive pulley.
In this case, the axial thrust of the driven pulley is determined by the belt transmission torque (torque transmitted between the pulleys) and the transmission ratio, the pulley axial thrust ratio of the drive side and driven side is found from the target transmission ratio and the transmitted torque ratio, the deviation of the pulley axial thrust is found from feedback elements of the dynamic transmission characteristic and transmission ratio, and a value obtained by adding the pulley axial thrust deviation to the product of the driven pulley axial thrust and pulley axial thrust ratio is set as the drive axial thrust (hydraulic pressure).
However, in the case of control using a prior art transmission control device as above, there is the problem that, when control was exercised such as to change the transmission ratio abruptly in the increasing direction as in the case of "kick-down", the pulley axial thrust deviation becomes a large negative value, causing a large drop in the drive pulley axial thrust and so giving rise to the possibility of belt slippage. In order to avoid this problem of occurrence of belt slippage, consideration has been given to setting the driven pulley axial thrust at a high level but if this is done the driven pulley axial thrust becomes unnecessarily large during steady running etc., giving rise to the problems of a lowering of efficiency of power transmission and an adverse effect in terms of fuel costs etc.